Thermal Protection Device

ABSTRACT

According to one aspect, a thermal protection device is provided having an electrical input for receiving input DC power and an electrical output for connection to a load. A heating circuit is coupled to the electrical input for developing one of first and second different heat magnitudes depending upon a temperature magnitude in the housing. An additional circuit is in thermal communication with the heating circuit for providing output DC power to the electrical output when the first heat magnitude is developed and for interrupting the output DC power when the second heat magnitude is developed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/728,761, filed Dec. 27, 2012, entitled “Thermal Protection Device”(Cree docket No. P1838US1).

REFERENCE REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable

SEQUENTIAL LISTING

Not applicable

BACKGROUND

1. Field of the Invention

The present invention relates generally to protection devices for loads,and more particularly to a thermal protection device for protecting aheat-producing load, such as an LED luminaire.

2. Description of the Background

Solid state light emitters including organic, inorganic, and polymerlight emitting diodes (LEDs) may be utilized as an energy-efficientalternative to more traditional lighting systems. Many modern lightingapplications utilize high power solid state emitters to provide adesired level of brightness. Generally, the lifetime of an LED isinversely related to the operating junction temperature thereof.Therefore, thermal management of the junction temperature is animportant design consideration of a luminaire (i.e., a lighting fixture)incorporating one or more LED's. Many solid state luminaires utilize aheat exchanger that dissipates heat into the ambient environment so asto obtain a desired life of the solid state emitters. However, suchluminaires can be installed in spaces containing insulation that canprevent the necessary airflow to maintain the junction temperature at orbelow an upper temperature limit. Because of this possibility, solidstate luminaires typically are provided with a thermal protectioncircuit that disconnects the LEDs from a power source when a sensedtemperature is reached. Often, such a circuit includes a bi-metal switchthat opens when the sensed temperature exceeds a threshold and/or aone-time thermal fuse that breaks a circuit connection when a fusingtemperature is reached.

LEDs operate more efficiently when powered by a direct current (DC)voltage rather than an alternating current (AC) voltage. Some knownsolid state luminaires utilize low power DC (i.e., at 60 volts orbelow). While low power DC solid state luminaires have some inherentadvantages (including the ability to control the fixture with low powercomponents and cabling), the use of low power DC can be problematic whendesigning a thermal protection circuit for the luminaire. Specifically,the lack of AC power at the fixture prevents the use of AC rated RP(thermal overload protection) devices and still meet UL1598requirements. Also, the cost of the AC RP devices has been increasing oflate due to the existence of few manufacturers. An off-the-shelf DCrated UL XAPX2/8 recognized thermal protection bi-metal switch istypically not specified for operation up to 60 VDC because such a deviceis too large and quite expensive. A UL XCMQ2/8 recognized TCO (thermalcutoff fuse) typically has a temperature difference between the fusingand the holding temperatures (i.e., the maximum normal operatingtemperature) that renders the device impossible to use with typicalluminaire maximum operating temperatures.

SUMMARY

According to one aspect, a thermal protection device is provided havingan electrical input for receiving input DC power and an electricaloutput for connection to a load. A heating circuit is coupled to theelectrical input for developing one of first and second different heatmagnitudes depending upon a temperature magnitude in the housing. Anadditional circuit is in thermal communication with the heating circuitfor providing output DC power to the electrical output when the firstheat magnitude is developed and for interrupting the output DC powerwhen the second heat magnitude is developed.

According to a further aspect, a thermal protection device is providedhaving a housing, an electrical input for receiving input DC power, andan electrical output for connection to a load. A heating circuit isdisposed in the housing and coupled to the electrical input and developsfirst and second different heat magnitudes depending upon a temperaturein the housing. An additional circuit is operative to provide output DCpower to the electrical output when the first heat magnitude isdeveloped and to interrupt the output DC power when the second heatmagnitude is developed.

According to yet another aspect, a thermal protection device is providedhaving a housing, an electrical input for receiving input DC power at avoltage magnitude up to 60 volts, and an electrical output forconnection to a load. A heating circuit is disposed in the housing andcoupled to the electrical input and includes first and secondseries-connected heating resistors and a thermal switch coupled inparallel across the second heating resistor, wherein the thermal switchcauses the heating circuit to develop a first heat magnitude when thethermal switch is exposed to a temperature in the housing below athreshold temperature and causes the heating circuit to develop a secondheat magnitude greater than the first heat magnitude when the thermalswitch is exposed to the temperature in the housing above the thresholdtemperature. An additional circuit is disposed in the housing andcoupled to the heating circuit and provides output DC power to theelectrical output when the first heat magnitude is developed andinterrupts the output DC power when the second heat magnitude isdeveloped, wherein the additional circuit includes a thermal circuitbreaker having a holding temperature greater than the thresholdtemperature and a fusing temperature greater than the holdingtemperature.

Although not so limited, the present invention finds particular utilityin one or more aspects of providing thermal protection for a DC load,such as an LED luminaire, so that an over-temperature condition isavoided.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the present invention will become evident by areading of the attached specification and inspection of the attacheddrawings in which:

FIG. 1 is an isometric view of a luminaire incorporating a thermalprotection device according to the present invention;

FIG. 2 is an enlarged, fragmentary, perspective view of a portion of theluminaire of FIG. 1 showing the thermal protection device in greaterdetail;

FIG. 3 is an exploded isometric view of an embodiment of the presentinvention;

FIGS. 4 and 5 are front and back elevational views of a circuit boardillustrating the mounting and positioning of circuit elements of theembodiment of FIGS. 1-3 thereon; and

FIG. 6 is a schematic diagram of the circuit mounted on the circuitboard of FIGS. 4 and 5.

DETAILED DESCRIPTION

Referring first to FIGS. 1-3, a thermal protection device 11 includescomponents disposed in a housing 12 comprising housing portions 12 a and12 b. The housing portions 12 a, 12 b may be secured together in anysuitable fashion, such as by fasteners comprising rivets or screws 14 a,14 b. The housing 12 is adapted to be secured in any known fashion to aluminaire 16, such as an LED luminaire as seen in FIG. 2. In theillustrated embodiment, the housing 12 includes a threaded portion 18(including threaded sections 18 a, 18 b) that may be inserted into aknock-out opening of a junction box 19. The threaded portion may receivea threaded nut (not shown) that secures the housing 12 to the junctionbox 19. The housing includes openings comprising vent holes 20 a, 20 bin the housing portions 12 a, 12 b, respectively, and a further openingcomprising vent hole 22 in the portion 12 a that permit air flow intothe housing 12 such that the components therein are disposed in heattransfer relationship with the ambient environment surrounding theluminaire 12, particularly when the luminaire 16 is installed at or in astructure, such as a ceiling, wall, or other surface. As is event to oneof ordinary skill, the space behind the ceiling, wall, or other surfaceoccupied by the luminaire 16 may be filled with thermal insulation,which may interfere with the dissipation of heat developed by theluminaire 16. The LED(s) used in the luminaire 16 may heat to damagingor destructive levels and/or the useful lifetime of the LED(s) may beundesirably shortened if the luminaire 16 is not thermally protected.

Referring to FIG. 6, the thermal protection circuit 11 includes an input30 coupled to conductors 32 a, 32 b and an output 34 coupled to aconductor 36. The input 30 receives DC power from a wall-mounted switchand power converter circuit (not shown) and components comprising LEDsof the luminaire 12 are coupled to the output 34. The thermal protectioncircuit includes a heating circuit 38 comprising at least a firstheating element 40, a second heating element 42, and a thermal switch44. In the preferred embodiment, each of the first and second heatingelements 40, 42 is implemented by at least one heating resistor. In theillustrated embodiment, the first heating element 40 is implemented by afirst pair of parallel-connected heating resistors R1, R2 and the secondheating element 42 is implemented by a second pair of parallel-connectedheating resistors R3, R4. In addition, the thermal switch 44 preferablycomprises a bi-metal thermal switch. The heating circuit 38 develops afirst heat magnitude when the housing 12, and/or the components thereinis (are) exposed to a temperature below a threshold and further developsa second heat magnitude greater than the first heat magnitude whenexposed to a temperature above the threshold. This functionality isachieved by selecting a thermal switch 44 having a switching point atthe threshold temperature as noted in greater detail hereinafter suchthat the thermal switch is closed when exposed to a temperature belowthe threshold and is open when the temperature is above the threshold.As should be evident, the first heating element 40 develops heat whenthe thermal switch 44 is closed and the first and second elements 40, 42both develop heat when the thermal switch 44 is open. In the preferredembodiment, the switching point of the thermal switch 44 is betweenabout 65 degrees C. and about 70 degrees C., and is most preferablyequal to about 70 degrees C.

A thermal circuit breaker 45 is coupled between the elements 40, 42 andthe LEDs coupled to the output 34. Specifically, a controllable switchcomprising a MOSFET transistor Q1 is coupled between the heatingelements 40, 42 and a thermal fuse (in this case a thermal cut-out orTCO) 50. The TCO 50 is coupled to the output 34 and has a holdingtemperature and a trip or fusing temperature.

In the preferred embodiment, the holding temperature of the TCO 50 isabout 100 degrees C. Also, the trip or fusing temperature is betweenabout 119 degrees C. and about 125 degrees C., and is most preferablyequal to about 125 degrees C.

From the foregoing, it can be ascertained that the holding temperatureof the TCO 50 is preferably greater than the switching point of thethermal switch 44. Also, of course, the trip or fusing temperature isgreater than the holding temperature.

The controllable switch Q1 is operated by a control circuit 52 inaccordance with a negative temperature coefficient (NTC) signal so thatthe controllable switch Q1 can be controlled to interrupt current to theTCO 50 before the TCO trip or fusing temperature is reached.Specifically, referring to FIG. 6, under ordinary operating conditions(i.e., when an over-temperature condition is not being experienced), aresistor R8, a zener diode D2, and a capacitor C level shift and developa regulated voltage of 15 volts across the capacitor C and the zenerdiode D2. A voltage divider comprising resistors R10 and R11 receive the15 volt regulated signal and cause a voltage of less than 2.5 volts tobe provided to a precision adjustable shunt regulator integrated circuit(IC) 54. The IC 54 is thereby maintained in an off condition at thistime. In addition, a MOSFET transistor Q2 is on due to the biasingprovided by resistors R8 and R9, thereby maintaining the controllableswitch Q1 in the on state. The current through the transistor Q2 islimited by resistors R5 and R6, and the gate to source voltage of thecontrollable switch Q1 is limited by a zener diode D1. If thetemperature in the housing 12 rises above the threshold temperature ofthe thermal switch 44, the thermal switch 44 opens, thereby causingcurrent to flow through the resistors R3 and R4 and quickly increasingthe temperature within the housing 12. When a housing temperature of,for example, about 90 degrees C., is reached, the regulated voltagedelivered to the IC 54 rises above 2.5 volts, thereby turning IC 54 onand turning off transistor Q2 and the controllable switch Q1. The LEDsare thereby turned off and current flow through the resistors R1-R4 isinterrupted until the temperature in the housing drops to a furtherlevel of, for example, about 80 degrees C. At this point the voltagedelivered to the IC 54 drops below 2.5 volts, thereby turning off IC 54and turning on transistor Q2 and the controllable switch Q1. The LEDsare turned back on and current flow is reestablished through theresistors R1-R4. The cycle then repeats indefinitely until power isremoved from the luminaire. The LEDs thereby flash on and off withoutcausing the TCO 50 to trip, thereby indicating to an observer that anover-temperature condition is being encountered.

The components of the device 11 are carried by a circuit board 60 asseen in FIGS. 3-5. The circuit board 60 is secured inside the housing 12in any suitable fashion. Electrical connections to the inputs and outputare accomplished via a header connector 62 and/or card edge connectors63.

As is evident from the foregoing, the present invention utilizes a heatsource inside the housing 12 when the load is operational comprising oneor both of the two heating elements 40, 42. The current flowing throughthe first heating element produces heat that is transferred to thethermal switch 44 and the other components in the housing 12, includingthe TCO 50. The bi-metal thermal switch 44 is connected in parallel withone of the heating elements and is set to trip if the temperature insidethe housing 12 reaches a certain point. When the thermal switch triptemperature is reached the thermal switch opens and current thereafterflows through the second heating element 42, which increases the heatlevel inside the housing 12. Significantly, when the thermal switch 44opens, the switch 44 is only subjected to the current*resistance voltagedrop (V=I*R). For example, assume that a current of 700 mA is flowing inthe heating circuit 38 and that each heating element 40, 42 has anequivalent resistance of 3.4 ohms. The thermal switch 44 initiallyshorts out the resistors R3, R4 and when the switch 44 trips (i.e.,opens) the current transitions to the 3.4 ohm heating element 42 fromthe thermal switch 44. The switch 44 has to break 700 mA at a voltage of2.38 VDC, which is well within the specifications of off-the-shelfbi-metal switch devices. The second heating element 42 thereaftercreates additional heat in the housing 12 and the temperature thereinquickly rises. The use of the switch 44 opening and creating additionalheat allows the TCO fusing temperature to be increased and keep the TCOholding temperature out of the normal operational range of theluminaire. (The range between the holding temperature and the fusingtemperature (typically a 25 degree C. range) comprises a keep-out areain which the luminaire is prevented from operating). Eventually, thecontrollable switch Q1 is turned off before the TCO fusing temperatureis reached in the housing 12 so that the current is interrupted beforethe one-time TCO trips. On the other hand, if the controllable switch Q1fails to open before the fusing temperature is reached, the TCO trips,preventing power from reaching the LEDs of the luminaire. In eitherevent, given the heat flow pattern in the housing 12, this heat rise canonly occur if the airflow through the housing is blocked (e.g., wheninsulation is covering the vent holes 20, 22) or if ambient temperatureincreases past a certain point.

Because the TCO 50 is a one-time-only fuse it is protected by thecontrollable switch Q1 that is operated in accordance with a negativetemperature coefficient developed signal. Of significance is the factthat none of the solid state devices used in the present invention israted for thermal overload protection and therefore all are specifiedsolely on performance criteria. Further, the fact that the controllableswitch Q1 is in series with the resistors, TCO, and load facilitatessatisfaction of UL requirements to short and open the semiconductor toevaluate fault conditions. Specifically, the switch Q1 can be maintainedin the on condition such that the TCO is not protected and the internalhousing temperature may be allowed to rise to the TCO fusingtemperature. Conversely, the switch Q1 may be maintained in the offcondition so that no current will flow and no heat is developed in thehousing.

INDUSTRIAL APPLICABILITY

The present invention utilizes a combination of a change in convectivecooling due to insulation placement and a rapid increase in the housing.These cascading events permit the use of off-the-shelf UL rated reliablecomponents. The three main components are UL “reliable” power resistorsas the heating elements 40, 42, the bi-metal thermal switch (which maybe an XAPX2/8 device manufactured by Uchiya of Japan), and the TCO 50(which may be an XCMQ2/8 device manufactured by Xiamen Set ElectronicsCo., Ltd. of Xiamen, China). The combination of these three componentsallows for a UL recognized device. The use of the switch Q1 incombination with the heating elements and TCO allows the device tocomply with UL standards and further permits the circuit to shut off andcool down from high temperatures without causing the one-time TCO totrip (and thereby require servicing).

The present invention may be used in numerous applications, such as inEMI sensitive applications, such as MRI rooms, telephone central officesoperating on 48 VDC, and safety lighting systems operating on DC backup.The present invention operates at 60V DC and at useful constant currentinputs such as 525 mA and 1400 mA. New current levels can quickly beimplemented with only a resistor value change. The circuit can pass ULtests as a stand-alone device. The circuit does not require the heatingfrom the luminaire in order to operate. The circuit can mimic currentlyavailable AC thermal overload protector devices satisfying UL categoryXAPX2/8 specified ability to cycle on and off under thermal overloadconditions and meets UL safety requirements for recessed devices using25 VDC UL recognized bi-metal switches. The present invention furtheruses internal convective airflow to operate, and is not just a solidbody. The TCO fusing temperature can be increased to ensure that anover-temperature condition does not cause operation in the temperaturerange between the holding temperature and the fusing temperature.Further, a fault resulting in the controllable switch remaining onduring an over-temperature condition, as opposed to the intermittentoperation of the switch Q1 as discussed above, will result in rapidheating to the fusing temperature and subsequent tripping of the TCO 50.Therefore, the time to trip in fault conditions is decreased.

As should be evident to one of ordinary skill in the art, the presentinvention can be implemented by other embodiments, structures, and/orprocesses. While the present invention is useful to provide thermalprotection for a DC powered LED luminaire, the present invention canprovide thermal protection for other loads that may or may not receiveDC power.

Numerous modifications to the present invention will be apparent tothose skilled in the art in view of the foregoing description.Accordingly, this description is to be construed as illustrative onlyand is presented for the purpose of enabling those skilled in the art tomake and use the invention and to teach the best mode of carrying outsame. The exclusive rights to all modifications which come within thescope of the appended claims are reserved.

We claim:
 1. A thermal protection device, comprising: an electricalinput for receiving input DC power; an electrical output for connectionto a load; a heating circuit coupled to the electrical input fordeveloping one of first and second different heat magnitudes dependingupon a temperature magnitude in the housing; and an additional circuitin thermal communication with the heating circuit for providing outputDC power to the electrical output when the first heat magnitude isdeveloped and for interrupting the output DC power when the second heatmagnitude is developed.
 2. The thermal protection device of claim 1,further including a thermal circuit breaker in thermal communicationwith the heating circuit.
 3. The thermal protection device of claim 2,wherein the heating circuit includes a thermal switch that causes theheating circuit to develop the first heat magnitude when the thermalswitch is exposed to a temperature below a threshold temperature andthat causes the heating circuit to develop the second heat magnitudewhen the thermal switch is exposed to a temperature above the thresholdtemperature.
 4. The thermal protection device of claim 3, wherein thethermal circuit breaker has a holding temperature greater than thethreshold temperature and a fusing temperature greater than the holdingtemperature.
 5. The thermal protection device of claim 3, wherein theheating circuit comprises a first heating element and a second heatingelement wherein the thermal switch is coupled in parallel with thesecond heating element.
 6. The thermal protection device of claim 5,wherein the thermal switch comprises a bi-metal device having aswitching point at the threshold temperature.
 7. The thermal protectiondevice of claim 1, wherein the additional circuit includes acontrollable switch.
 8. The thermal protection device of claim 7,wherein the controllable switch is operated in accordance with anegative temperature developed signal.
 9. The thermal protection circuitof claim 1, wherein the electrical input is adapted to receive DC powerup to 60 volts.
 10. A thermal protection device, comprising: a housing;an electrical input for receiving input DC power; an electrical outputfor connection to a load; a heating circuit disposed in the housing andcoupled to the electrical input and that develops first and seconddifferent heat magnitudes depending upon a temperature in the housing;and an additional circuit operative to provide output DC power to theelectrical output when the first heat magnitude is developed and tointerrupt the output DC power when the second heat magnitude isdeveloped.
 11. The thermal protection device of claim 10, furtherincluding a thermal circuit breaker disposed in the housing.
 12. Thethermal protection device of claim 11, wherein the heating circuitincludes a thermal switch that causes the heating circuit to develop thefirst heat magnitude when the temperature in the housing is below athreshold temperature and that causes the heating circuit to develop thesecond heat magnitude when the temperature in the housing is above thethreshold temperature.
 13. The thermal protection device of claim 12,wherein the thermal circuit breaker has a holding temperature greaterthan the threshold temperature and a fusing temperature greater than theholding temperature.
 14. The thermal protection device of claim 13,wherein the heating circuit comprises a first heating element and asecond heating element wherein the thermal switch is coupled in parallelwith the second heating element.
 15. The thermal protection device ofclaim 14, wherein the thermal switch comprises a bi-metal device havinga switching point at the threshold temperature.
 16. The thermalprotection device of claim 15, wherein the additional circuit includes acontrollable switch.
 17. The thermal protection device of claim 16,wherein the controllable switch is operated in accordance with anegative temperature developed signal.
 18. The thermal protectioncircuit of claim 10, wherein the electrical input is adapted to receiveDC power up to 60 volts.
 19. A thermal protection device, comprising: ahousing; an electrical input for receiving input DC power at a voltagemagnitude up to 60 volts; an electrical output for connection to a load;a heating circuit disposed in the housing and coupled to the electricalinput and including first and second series-connected heating resistorsand a thermal switch coupled in parallel across the second heatingresistor, wherein the thermal switch causes the heating circuit todevelop a first heat magnitude when the thermal switch is exposed to atemperature in the housing below a threshold temperature and causes theheating circuit to develop a second heat magnitude greater than thefirst heat magnitude when the thermal switch is exposed to thetemperature in the housing above the threshold temperature; and anadditional circuit disposed in the housing and coupled to the heatingcircuit and that provides output DC power to the electrical output whenthe first heat magnitude is developed and that interrupts the output DCpower when the second heat magnitude is developed, wherein theadditional circuit includes a thermal circuit breaker having a holdingtemperature greater than the threshold temperature and a fusingtemperature greater than the holding temperature.
 20. The thermalprotection device of claim 19, wherein the additional circuit furtherincludes a controllable switch operated in accordance with a negativetemperature developed signal.